1. Field of the Invention
The present invention relates to a dielectric ceramic body which is adapted to be used for a ceramic bandpass filter. More particularly, the present invention pertains to a dielectric ceramic body of a substantially cuboidal configuration having a conductive or short-circuited surface which is coated with a layer of a conductive material and an open surface which is opposite to the conductive surface and is not coated with a conductive material, one of the conductive and open surfaces being formed with a plurality of spaced apart slits which determine coupling properties in the body.
2. Description of the Prior Art
Conventionally, a ceramic filter has been structured to include a plurality of cylindrical ceramic bodies which are arranged in series and electrically connected to provide a transmitter resonator and a receiver resonator, respectively. This type of ceramic filter is disclosed for example by the U.S. Pat. No. 4,255,729 issued to A. Fukasawa et. al. on Mar. 10, 1981. The known type of ceramic filter is found disadvantageous in that it requires a lot of labors in manufacture.
The U.S. Pat. No. 3,505,618 issued to F. B. L McKee on Apr. 7, 1970 discloses a filter which is made of a dielectric body of a substantially cuboidal configuration. The body is formed with a plurality of through holes which are arranged in series and extend from one surface of the body to the opposite surface. The body is covered by a layer of a conductive material such as silver. The holes have surfaces which are covered by a conductive material and conductive terminals may be inserted into selected holes to provide an input and an output.
The patent proposes to provide the body with polystyrene, however, the body may be made of a dielectric ceramic material as disclosed by the U.S. Pat. No. 4,464,640 issued to T. Nishikawa et. al. on Aug. 7, 1984. In the filter structure proposed by Nishikawa et. al., the ceramic body is covered by a conductive material except a surface where one ends of the holes are opened. The surface which is not covered by the conductive material may be referred as the open surface and the surface which is covered by the conductive material and opposite to the open surface may be referred as the short-circuited surface. In order to provide a desired coupling between each two adjacent holes, a slit is formed between the holes. As shown in the U.S. Pat. No. 4,431,977 issued to R. L. Sokola et. al. on Feb. 14, 1984, the ceramic body may be of an elongated configuration and a plurality of holes may be formed and arranged in series in the axial direction of the body. In the structure as shown in the patent to Sokola et. al., a slit is formed between each two adjacent pair of the holes.
This type of ceramic filters are useful in that it can be readily assembled and less expensively manufactured. The number of the holes may be appropriately determined to obtain a desired function. In this type of ceramic filter, a desired resonating frequency is obtained through an adjustment of the volume of the dielectric material between the conductive layers on the surfaces of each two adjacent holes or the conductive layer on the surface of each hole and the outer surface of the ceramic body. For the purpose, the thickness of the ceramic body or the distance between the open surface and the short-circuited surface is appropriately changed to thereby change the lengths of the holes. For example, atan axially end portion, the thickness of the ceramic body may be reduced to provide a shortened hole length or a piece of dielectric material may be added to an appropriate portion of the ceramic body.
It should however be noted that the conventional method for obtaining a desired resonating frequency is inconvenient in that the number of parts and the labor for the manufacture are undesirably increased. The patent to Sokola et. al. proposes to cover even the surface opposite to the short-circuited surface except the area around each hole to provide a desired coupling. It should however be noted that the structure as proposed by Sokola et. al. is not satisfactory, either.